Generally, Hall devices are used for the rotation control and position detection of motors and for magnetic field detection, and find extensive application in such fields as brushless motor, noncontact open/close detection switch, and current sensor.
Recently, mobile equipment typified by mobile phones, mobile information terminals and notebook personal computers has become widely available. The Hall devices are widely applied not only to such devices that are installed into the mobile equipment, but also to technical fields of automobiles and the like.
For example, the Hall device is used as a pointing device for reading an input direction and an amount of movement by detecting the movement of a magnet mounted on an input portion. This pointing device has an advantage over a button pointing device whose input information is only a direction in that it can input more detailed information including both the direction and amount of movement.
Furthermore, the Hall device is also applicable as a sensor used for an open/close detection switch of a flap attached to the mobile equipment. The sensor includes a magnet placed at one of the body and flap, and a Hall device placed at the other of them, and detects the opening and closing according to the variations in the magnetic field involved in the movement of the magnet. The open/close detection switch using the Hall device has an advantage over open/close detection switches with contacts such as a reed switch in that its life is semipermanent.
Moreover, the Hall device is also used for a geomagnetic direction sensor.
Basic characteristics required of the Hall device used for the mobile equipment are low power consumption, high sensitivity, and good temperature characteristics. The low power consumption is the most important characteristics of the mobile equipment with a battery.
As for the sensitivity, the higher sensitivity makes it possible to detect smaller variations in the magnetic field, thereby increasing flexibility in placing the magnet and Hall device, and in selecting the type of the magnet. As for the temperature characteristics, the better temperature characteristics makes it possible to achieve better accuracy without adding any extra function to the circuit such as that for temperature correction, thereby simplifying the circuit design and reducing the cost.
In such technical fields, magnetic sensors with low power consumption, high sensitivity and excellent temperature characteristics are required. In particular, it is very important for the mobile equipment such as mobile phones to have low power consumption.
Generally, the major characteristics of the Hall device constituting the magnetic sensor depend greatly on the characteristics of the semiconductor material. For example, the sensitivity is proportional to the electron mobility of the semiconductor material, and the power consumption, which is inversely proportional to the input resistance of the device, becomes smaller as the sheet resistance of the semiconductor material increases.
The conventional Hall devices use compound semiconductors with large electron mobility, particularly such as InAs, InSb and GaAs. This is because the InAs, InSb and GaAs and their compound semiconductors have high electron mobility, and are suitable as a material for the highly sensitive Hall devices. Although the Hall devices using the InSb or InAs have good sensitivity, they have a problem of being inferior in the temperature characteristics and power consumption characteristics. It is known that the temperature characteristics of the Hall devices can be improved by doping Si into InAs. However, they cannot satisfy other device characteristics such as the sensitivity characteristics and power consumption characteristics. In contrast, although the Hall devices using the GaAs as the material have good temperature characteristics and power consumption characteristics, they have a problem in that their sensitivity is low.
Thus, no Hall devices are applied easily to the mobile equipment. Accordingly it is necessary to use the Hall devices with various of contrivances.
As for these problems, Japanese Patent No. 3069545 discloses a stacked structure having a first compound semiconductor layer, an InAs layer formed thereon as an active layer, and a second compound semiconductor layer with a high resistance formed on the top surface of the InAs layer. It forms a quantum well potential in the InAs active layer, increases the mobility of the electrons passing through the active layer and the sheet resistance because of the appearance of the quantum effect, thereby being able to form a stacked structure with good temperature characteristics.
Japanese Patent No. 2793440 (Japanese Patent Application Laid-open No. 6-077556 (1994) discloses that a high electron mobility, large input resistance and excellent temperature characteristic Hall device can be formed by forming an InAs active layer on an AlGaAsSb layer that has a lattice constant close to that of the InAs active layer and a large band gap.
In addition, J. Vac. Sci. Technol. B16(1998) p. 2644 discloses a structure achieving higher electron mobility by forming an AlGaAsSb layer on InAs, with a structure sandwiching an InAs active layer with compound semiconductor layers with a large band gap.
As a technique concerning the device structure of the Hall device that sandwiches the InAs active layer with the AlGaAsSb layers, there is Japanese Patent Application Laid-open No. 9-116207 (1997), for example. It describes that the reliability is improved by removing all the semiconductor thin films other than a magneto-sensitive pattern, and by covering the entire magneto-sensitive pattern and electrode layer with a passivation. It also describes that the electrode layer may make contact either with the InAs layer or with the upper AlGaAsSb layer.
However, to bring the sensitivity and resistance value of the Hall devices employing such a compound semiconductor stacked structure into specified design ranges, it is necessary to place the electron mobility and sheet resistance value of the compound semiconductor material within certain ranges with high reproducibility. However, since it is difficult to control their physical property values, a problem arises in that it is difficult to manufacture the Hall devices with the quantum well compound semiconductor stacked structure industrially.
In addition, the foregoing Hall device has a problem in that the absolute value of an unbalanced voltage (Vu) does not agree with that of a rotated unbalanced voltage (rVu) obtained by rotating the input and output terminals by 90°. In a linear Hall IC used as a current sensor, an output voltage obtained by the 90° rotation is added to cancel out the Vu, thereby achieving good linearity of the Hall output for the magnetic field. It is desirable that the absolute value of the Vu agree with that of the rVu, and the difference between the absolute values of the Vu and rVu must be within 0.5 mV (for a input voltage of 3V) from a practical standpoint. Therefore it is required that the disagreement between the foregoing Vu and rVu (called Vu+rVu variations from now on) be improved.
Furthermore, the foregoing Hall devices have a problem in that their reliability is insufficient. More specifically, the unbalanced voltage (Vu) value varies greatly under high temperature, high moisture conditions, and input resistance (Rin) and unbalanced voltage (Vu) value vary greatly when soldering the Hall device to a printed circuit board.
FIG. 1 is a view showing an example of a Hall device with a structure disclosed in Japanese Patent Application Laid-open No. 9-116207 (1997), and FIG. 2 is a flowchart illustrating its fabrication process. In FIG. 1, the reference numeral 1 designates a substrate, 2 designates semiconductor thin films, 2a designates a first compound semiconductor layer, 2b designates an active layer, 2c designates a second compound semiconductor layer, 2d designates a third compound semiconductor layer, 3 designates a metal electrode layer, and 4 designates a passivation.
The Hall device has the metal electrode layer formed by a lift off method, and is completed by covering the entire device with the passivation, followed by opening a bonding pad section. The reason for using the lift off method for forming the electrodes is that the compound semiconductor including Sb has not enough resistance to acid and alkali, and hence it is difficult to carry out patterning by etching using the acid or alkali after forming the metal electrode layer on the entire surface.
However, the metal electrode layer formed by the lift off method has a problem of leaving protruding, ragged edges (burrs) at the edges of the patterns, thereby greatly deteriorating the covering conditions of the passivation formed thereon. As a result, a high temperature, high moisture accelerated test results in corrosion of the compound semiconductor including Sb with insufficient moisture resistance, thereby varying the unbalanced voltage Vu greatly.
Because of these variations, the conventional Hall devices are not practical. Thus, the Hall devices are required with small characteristic variations under the high temperature, high moisture conditions and due to soldering, that is, with a high reliability.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to ensure a stable supply of a stacked structure of a quantum well compound semiconductor with high electron mobility and sheet resistance and with excellent temperature characteristics by improving the reproducibility of controlling the physical properties of the quantum well compound semiconductor stacked structure, thereby enabling industrial provision of highly sensitive, low power consumption Hall devices with excellent temperature characteristics.
Another object of the present invention is to provide a compound semiconductor Hall device with small Vu+rVu variations in the stacked compound semiconductor Hall device capable of achieving high sensitivity using InAs or the like as an active layer.
Still another object of the present invention to provide a highly reliable compound semiconductor Hall device and its fabrication method in the stacked compound semiconductor Hall device capable of achieving high sensitivity using InAs or the like as an active layer.
Still another object of the present invention to provide a magnetic sensor suitable for various devices used for the mobile equipment such as pointing devices.